1. Field of the Invention
The present invention relates to an image forming apparatus which records, onto recording mediums, images formed with image forming means and also forms images by rotating a belt being supported by rollers. More particularly, the present invention relates to an image forming apparatus including a means for correcting the curling tendency of the belt.
2. Description of the Related Art
As image forming apparatuses such as copying machines and printers, electrophotographic type image forming apparatuses have been widely used as image forming means. In the present day, apparatuses capable of forming color images have been used as such copying machines.
An electrophotographic type color-image forming apparatus includes plural image forming stations for forming images in respective colors of yellow, magenta, cyan and black which are arranged in parallel and superimposes images formed in the respective image forming stations to form a color image. As the configuration for superimposing the images having the respective colors, an intermediate transfer belt which is an endless belt is placed such that it is faced to the plural image forming stations placed in parallel, and images formed in the respective image forming stations are successively primarily transferred to the intermediate transfer belt to form a color image. This color image is secondarily transferred to a recording medium at a secondary transfer portion and, then, the color image is output.
Also, in addition to the aforementioned intermediate transfer system, there is a system for conveying a recording medium through an endless belt facing to respective image forming stations and successively transferring color images formed in the respective image forming stations to the recording medium such that they are superimposed thereon to form a color image.
As described above, a color-image forming apparatus forms images by rotating an endless belt, wherein the endless belt is wound around and supported by plural rollers. However, bending stresses are constantly generated in the endless belt wound around and supported by the tension rollers, at its portions which come into contact with the tension rollers.
Usually, such bending stresses and physical changes are dispersed over the entire belt during movement of the belt, which prevents the occurrence of significant malfunctions. However, when the belt is temporarily brought into a standby state, such bending stresses and physical changes are generated concentratively at the portions of the belt which come into contact with the rollers and the heated portion of the belt. This may cause permanent sets in the belt, thereby causing image degradations such as color shifts, color heterogeneity and the like.
Also, it is possible to employ a method which idly rotates the intermediate transfer belt for a predetermined time period after the start of operation in order to disperse bending stresses and physical changes over the entire belt. However, if the standby time period becomes longer, this will make it impossible to form images efficiently.
Therefore, there has been disclosed a configuration for releasing or reducing the pressures exerted on a belt from tension rollers at non-operation states of an apparatus, in order to prevent the aforementioned belt from exhibiting a curling tendency (Japanese Patent Application Laid-open No. 2003-173090).
However, if the apparatus is configured to release or reduce the pressures exerted on the belt from the tension rollers at non-operation states of the apparatus as described in the aforementioned Japanese Patent Application Laid-open No. 2003-173090, this will cause the problem of increases of the complicacy and the size of the apparatus.